I've been a die-hard user since WP 5 for DOS, and have had WP X4 for several months.

Things that are new in the latest edition:
Corel WP Lightning - It's basically a clipboard that saves notes in WP or Word format.

In terms of X4, the biggest legal-side change is the redaction feature. I don't play with that much, but those who I've heard have say it works fairly well.

The metadata removal function works well. The equivalent Word feature is in Word 2007, but not in 2003 and earlier unless you download a plugin.

Most of the changes were to PDF. Editing PDF's are still hit or miss in my experience.

File format remains the same as it has forever... so no problems there. Will you find much of a difference between 12 and X4? Depends on how much you use PDF's. Printing to PDF is better in X4 than in previous versions - X4 finally adds some basic accessibility features.

Here's Corel's page on what's new.

You'll also find a wonderful WP community (if you haven't already) at wpuniverse.com.